Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session B21: Focus Session: Graphene: Magnetic Properties |
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Sponsoring Units: DMP Chair: Sudip Chakravarty, University of California, Los Angeles Room: Portland Ballroom 251 |
Monday, March 15, 2010 11:15AM - 11:27AM |
B21.00001: Why isn't graphene a strongly correlated electron system? P. Abbamonte, J.P. Reed, B. Uchoa, Y.I. Joe, E. Fradkin, D. Casa Graphene is usually described as a two-dimensional system of weakly interacting, Dirac fermions. The strength of the interactions is characterized by the so-called ``fine structure constant", $\lambda = e^2/\hbar v_F \epsilon$, which if $\epsilon \sim 1$ is $\sim 2$, indicating the absence of a small parameter for EM interactions and the presence of strong correlations. However, the salient signatures of correlations, such as spectral weight redistribution with doping, are not observed in graphene. To find out why, we used inelastic x-ray scattering to measure the collective excitations in single crystal graphite. Using some new data analysis methods, which ``remove" the van der Waals coupling between the layers, we reconstructed the density propagator for graphene, and then determined the dressing around an idealized point charge, $e$. The dressing is $\sim 2$ nm in size and renormalizes the charge to $e_{eff} = 0.065 e$ greatly suppressing interactions. This suggests that, for length scales $> 2$ nm, graphene can be thought of as having a ``background" dielectric constant $\epsilon = 15.4$, or a fine structure constant $\lambda \sim 0.14$. Our result explains the absence of spectral weight transfer with doping in graphene, the absence of a velocity divergence near $E_F$, and a number of other effects. [Preview Abstract] |
Monday, March 15, 2010 11:27AM - 11:39AM |
B21.00002: Spin and Valley Polarization on deep Coulomb States in Graphene Valeri Kotov, Vitor Pereira, Bruno Uchoa, Antonio Castro Neto In gapped graphene, electrons occupying the lowest energy level exhibit valley and spin degeneracies. The electrons are localized on the scale of the effective Compton wavelength, which is inversely proportional to the gap. We show that taking into account electron-electron interactions can lift both the spin and valley degeneracies, thus inducing spin and valley moments. Consequently the electrons can be polarized on nanometer (gap dependent) scale. We also explore the possible signatures of this effect, such as the modification of the density of states around the Coulomb center. [Preview Abstract] |
Monday, March 15, 2010 11:39AM - 11:51AM |
B21.00003: Magnetic Impurities In Graphene Feiming Hu, Tianxing Ma, Haiqing Lin, James E. Gubernatis We present a quantum Monte Carlo study of magnetic atoms embedded in a graphene sheet. We consider the hybridization between magnetic impurity atoms and carbon atoms in the graphene, which can be well described by the Anderson impurity model. We calculate the magnetic momentum and spin susceptibility of the impurity by tuning Coulomb interactions, temperature and chemical potentials. Furthermore the effective momentum (screened momentum) is studied in detail by correlation functions between the impurity and carbon atoms. Since system's magnetic and conducting properties depend on the arrangements of impurities, we investigate various kinds of impurity embedding configurations and make comparisons among them. [Preview Abstract] |
Monday, March 15, 2010 11:51AM - 12:27PM |
B21.00004: Magnetism in Graphene Invited Speaker: As a two-dimensional sheet of carbon atoms, graphene seems so far stable against all kinds of electronic instabilities. When adatoms are chemically adsorbed on top of graphene, on the other hand, there is a rich number of possibilities which so far remain little explored both theoretically and experimentally. One promising possibility is about formation, detection and manipulation of local magnetic moments and the subsequent emergence of controllable macroscopic magnetic states in graphene. In this talk I will focus in the problem of formation and detection of local magnetic moments by transport and STM probe measurements. In addition, I will also discuss recent Monte Carlo results for a disordered distribution of magnetic adatoms in graphene and explore the distinctive signatures of magnetism in the transport. Having in mind that the adsorption energy of the adatoms can change substantially according to the local curvature of the graphene sheet, the magnetic adatoms may not be randomly distributed, but might cluster around the top of the ripples. We propose that the interplay between the correlation induced by the ripples and by the RKKY interactions can generate a variety of magnetic states in graphene, with distinctive signatures in the magnetization and magnetoresistance curves. [Preview Abstract] |
Monday, March 15, 2010 12:27PM - 12:39PM |
B21.00005: Experimental Evidence of Many-Body Interactions in the Optical Spectra of Graphene Kin Fai Mak, Jie Shan, Tony Heinz Excitonic effects are usually considered to be unimportant in the optical response of metals because of strong carrier screening. Recent studies have, however, shown that such screening is significantly reduced in metallic systems of lowered dimensionality, such as carbon nanotubes [F. Wang et al., PRL 99, 227401 (2007)]. Excitonic effects have also been predicted to be present for interband transitions in graphene, a model 2-dimensional semimetal [L. Yang et al., PRL 103, 186802 (2009)]. We report here an experimental determination of the absorption spectrum of high-quality exfoliated graphene for a photon energies of 0.2 - 5.5 eV. A pronounced absorption peak is observed at 4.6 eV, with a gradual rise in absorbance from the universal value of $\pi \alpha $ on the low-energy side and a sharp decrease on the high-energy side. While increased absorption is expected to occur in the UV because of trigonal warping of the graphene band structure, both the position and the shape of the observed absorption peak provide evidence of pronounced excitonic effects in graphene. [Preview Abstract] |
Monday, March 15, 2010 12:39PM - 12:51PM |
B21.00006: Positive magnetic susceptibility in graphene Joel Therrien, Kyle Twarowski Magnetic force microscopy imaging of exfoliated graphene on oxidized silicon substrates was performed. In the magnetic force domain, an attractive force was observed whenever the magnetized tip was over a graphene sample on the substrate. Were the graphene showing diamagnetism as seen in HOPG, the observed force would have been repulsive. No attractive potential was observed on the graphene-free regions. The same regions were also scanned with a non-magnetic probe to check for possible electrostatic forces. None were found. Following up on these measurements, a flake of graphene was placed on the end of a tipless AFM cantilever. Using the AFM to measure any deflections in the cantilever, a magnetic field was applied with measureable deflections observed. ICP-MS analysis of the source graphite revealed magnetic impurities on the level of 8ppm. These results will be discussed in relation to defects within the graphene. [Preview Abstract] |
Monday, March 15, 2010 12:51PM - 1:03PM |
B21.00007: Room-temperature ferromagnetism in graphite driven by two-dimensional networks of point defects Kees Flipse, Jiri Cervenka, Misha Katsnelson Understanding the mechanism of ferromagnetism in carbon-based materials, which contain only s and p electrons in contrast to traditional ferromagnets based on 3d or 4f electrons, is challenging. Here, we demonstrate direct evidence for ferromagnetic order locally at defect structures in highly oriented pyrolytic graphite (HOPG) with magnetic force microscopy and in bulk magnetization measurements at room temperature. Magnetic impurities have been excluded as the origin of the magnetic signal. The observed ferromagnetism has been attributed to originate from localized electron states at grain boundaries of HOPG, forming two-dimensional arrays of point defects. The theoretical value of the magnetic ordering temperature based on weak interlayer coupling and/or magnetic anisotropy is comparable to the experimental value. The unusual chemical environment of defects bonded in graphitic networks can reveal the role of the s and p electrons, creating new routes for spin transport in carbon-based materials. [Preview Abstract] |
Monday, March 15, 2010 1:03PM - 1:15PM |
B21.00008: Theory of Fano Resonances in Graphene: The Kondo effect probed by STM Alexander Lichtenstein, Tim Wehling, Hari Dahal, Mikhail Katsnelson, Hari Maoharan, Alexander Balatsky We consider the theory of Kondo effect and Fano factor energy dependence for magnetic impurity (Co) on graphene. We have performed a first principles calculation and find that the two dimensional E{\_}1 representation made of d{\_}xz, dyz orbital is likely to be responsible for the hybridization and ultimately Knodo screening for cobalt on graphene. There are few high symmetry sites where magnetic impurity atom can be adsorbed. For the case of Co atom in the middle of hexagon of carbon lattice we find anomalously large Fano q-factor and strongly suppressed coupling to conduction band. This anomaly is striking example of quantum mechanical interference related to the Berry phase inherent to graphene band structure. [Preview Abstract] |
Monday, March 15, 2010 1:15PM - 1:27PM |
B21.00009: Orbitally controlled Kondo effect of Co ad-atoms on graphene Tim Wehling, Alexander Balatsky, Mikhail Katsnelson, Alexander Lichtenstein, Achim Rosch Based on ab-initio calculations we identify possible scenarios for the Kondo effect due to Co ad-atoms on graphene. For a Co atom absorbed on top of a carbon atom, the Kondo effect is quenched by spin-orbit coupling below an energy scale of $\sim \!\!15$\,K. For Co with spin $S=1/2$ located in the center of a hexagon, an SU(4) Kondo model describes the entanglement of orbital moment and spin at higher energies, while below $\sim 60$\,meV spin-orbit coupling leads to a more conventional SU(2) Kondo effect. The interplay of the orbital Co physics and the peculiar band-structure of graphene is directly accessible in Fourier transform tunneling spectroscopy or in the gate-voltage dependence of the Kondo temperature displaying a very strong, characteristic particle-hole asymmetry. [Preview Abstract] |
Monday, March 15, 2010 1:27PM - 1:39PM |
B21.00010: Magnetization of Graphane by Dehydrogenation Hasan Sahin, Can Ataca, Salim Ciraci Using first principles calculations, we show that each hydrogen vacancy created at graphane surface results in a local unpaired spin. For domains of hydrogen vacancies the situation is, however complex and depends on the size and geometry of domains, as well as whether the domains are single- or double-sided. In single-sided domains, hydrogen atoms at the other side are relocated to pair the spins of adjacent carbon atoms by forming $\pi $-bonds. Owing to the different characters of exchange coupling in different ranges and interplay between unpaired spin and the binding geometry of hydrogen, vacancy domains can attain sizable net magnetic moments. [Preview Abstract] |
Monday, March 15, 2010 1:39PM - 1:51PM |
B21.00011: Carrier velocity reduction in magnetic graphene superlattices Liang Zheng Tan, Cheol-Hwan Park, Steven Louie We investigate the effects of a periodic magnetic field modulation (superlattice) on the electronic structure of graphene using the effective (Dirac-Weyl) Hamiltonian approach, as well as tight-banding calculations. Using a Lorentz transformation of complex rapidity, we show that the low-energy electronic structure of graphene under a one-dimensional inhomogeneous magnetic field can be mapped into that of graphene under an electric field. The isotropic velocity reduction in magnetic graphene superlattices of zero average flux follows as a result of this transformation. The changes in the electronic structure introduced by the magnetic superlattice have important implications for the transport properties of this system. This work was supported by the NSF under Grant No. DMR07-05941, and the U.S. DOE under Contract No. DE-AC02-05CH11231. Computer time was provided by NERSC and NPACI. [Preview Abstract] |
Monday, March 15, 2010 1:51PM - 2:03PM |
B21.00012: Magnetism and Correlations of Fractionally Filled Zero-energy States in Graphene Quantum Dots A. Devrim Guclu, Pawel Potasz, Oleksandr Voznyy, Marek Korkusinski, Pawel Hawrylak We study electronic and magnetic properties of triangular graphene dots with zig-zag edges. Such structures have recently attracted attention due to the existance of a shell of degenerate states at the Fermi level, with half-filled shell exhibiting a magnetic moment[1,2,3]. In this work, we present new results demonstrating the important role of electronic correlations as a function of the filling fraction of the shell. The effect of degeneracy, finite size and electron-electron interactions are treated nonperturbatively using a combination of density functional theory, tight-binding, Hartree-Fock and configuration interaction methods. We show that the nature and magnetization of the ground state depend strongly on the filling fraction of the degenerate shell. Half-filled charge neutral shell leads to full spin polarization but this magnetic moment can be completely destroyed by adding a single electron. [1] J. Fernandez-Rossier and J.J. Palacios, Phys.Rev.Lett. {\bf 99}, 177204 (2007) [2] M. Ezawa, Phys.Rev.B, {\bf 77}, 155411 (2008) [3] Kaxiras {\it et al.}, Nano Lett. {\bf 8}, 241 (2008). [Preview Abstract] |
Monday, March 15, 2010 2:03PM - 2:15PM |
B21.00013: Magnetism of Graphene Nanoribbons on Ni(111) Keisuke Sawada, Fumiyuki Ishii, Mineo Saito The magnetic ground state of zigzag-edge graphene nanoribbon (ZGNR) has an antiferromagnetic (AFM) structure with anti-parallel ferromagnetic (FM) chains at both edges. Recently, we found that the FM and noncollinear magnetic states can be achieved by carrier doping [1]. In this study, we clarify that the FM state also appears in the case of ZGNRs on Ni(111) by using first-principles calculations. We find that the magnetic moment of the edge C atom in the mono ZGNR on Ni(111) layer is very small due to the hybridization between electron of the ZGNR and d electron of the first Ni layer. On the other hand, in the case of the bilayer ZGNR on Ni(111), the magnetic moment of the edge C atom at the top layer maintains and the FM state is the ground state. So the bilayer structure is favorable from the viewpoint of spintronics application. \\[4pt] [1] K. Sawada et al., Nano Lett. 9, 269 (2009). [Preview Abstract] |
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